Checking VHF aerial

[30boat]

I know it's been discussed before but I can't find the threads anywhere so I'm asking again.How can an aerial be checked?I think mine is not working and before I get a new one I'd like to make sure I really have to.
Thanks

 

[SAWDOC]

Is your mast up?
Have you tried doing a radio check with your friendly local coastguard?

 

[30boat]

Yes,the mast is up.I tried a radio check and the results were confusing.I have two VHFs.An older Sailor and a DSC Simrad RD68.The Sailor worked on and off but the Simrad didn't.When I connected the radios to a SWR I got power from the Sailor but not from the Simrad wich is probably buggered.When I connected them to the spare aerial again the Sailor worked but the Simrad didn't.I'm almost sure that both the aerial and the Simrad are broken but I would like to test the aerial anyway.

 

[William_H]

Clearly with no forward power indicated on the SWR meter the Simrad is not transmitting. Check all the switches etc.
Antenna come in different styles. The longer 1 metre or more in a fibreglass tube usually show a DC conductivity from screen to centre.
A bare wire 17inch antenna will not show conductivity.
The best test however is comparison with another antenna which you have done. It indicates either the cable to the mast top has a failure (in a joint usually) or the antenna is dead.
If you get the antenna off you can cut around the base of the f/g tube and remove the tube. Look for corrosion or wires fallen off the components.
THe SWR measurement will indicate if the antenna is good. However with a longer cable of the thin lossy cable the losses in the cable will look like a good antenna. So check at or near the antenna.
good luck olewill

 

[richardm47]

I've got some vhf problems too. Could you say what an SWR is please? Still Working Radio? Starbd Wireless Repeater? Side Winder Release? Maybe I need one of those.

Couldn't be Short Wave Radio could it? why would you connect the dodgy vhfs to that?

 

[shaqra]

Hi
have a look at this page it may answer your question about SWR.

http://signalengineering.com/ultimate/antenna_basics.html

 

[fisherman]

Just as an aside, one VHF failure I get is when the inline fuse gets a bit of corrosion or verdigris on it. You receive OK, which only needs a half amp or so, when you transmit, about 7 amps, the connection breaks down.

 

[Salty John]

VSWR stands for Voltage Standing Wave Ratio. If your antenna and radio were perfectly matched and there was no cabling between them you would have a VSWR of 1:1. When you add manufacturing imperfections and an imperfect cable run a mismatch of greater or lesser degree is introduced. This mismatch gets somewhat significant when it reaches about 1.5:1 and gets really serious at 3:1 and can damage your radio at higher levels.
Some radios have a built in VSWR meter and won't allow you to transmit when VSWR is above a dangerous level.
If you use the right componants - antenna, cable, connectors - and they are properly installed and undamaged you won't lose sleep over what your VSWR is, unless you are a radio fanatic!
Also, if you are unfamiliar with the use of a VSWR meter you can get things wrong and convince yourself all is well when it isn't.
With a masthead whip antenna you need to check with a multi-meter that the outer shell and the centre pin on your connectors are not shorted and, if possible, that there is continuity between the centre pin at the antenna and at the radio and the same for the outer shell.
The whip antenna will usually show a dead short if you measure across its centre socket to outer body so the above tests must be done with the cable disconnected at both ends.
The best check is to take the radio to another antenna or another antenna to the radio.
Sometimes you can test the antenna by extracting the PL259 connector at the radio end so only the pin is connected and not the outer body. Tune to a radio station. Then push the connector all the way home and the reception should slightly improve. If it doesn't and actually worsens or goes away you have a faulty antenna system.

 

[davidpbo]

Also look at the gauge of the power cable to the radio as fisherman says voltage drop (accross fuse or cable) can cause a problem.

On my boat which has basic electrics running a few lights, radio and depth gauge I need to upgrade my cable from battery to fuse panel from 2.5mm. Whilst it can cope with the current demand (Fused at 15A I think) the voltage drop is too high and when I transmit cabin lights dim markedly.

On a metre long aerial like I have with a plastic tube/bulb at the bottom I get 1 OHm d.c. resistance between core and outer cable at or near the ariel. In my aerial and many others there is a centre tapped coil inside the tube which forms a short circuit to D.C. resistance but has high impedance (might be regarded as resistance to A.C.) at radio frequency.

Other radio buffs please take over I am out of my comfort zone!

 

[MarkGrubb]

If you want to check whether the Simrad is transmitting connect it to the power/SWR meter as you did, but terminate the output of the meter, which usually connects to the antenna, with a 50 ohm dummy load (can be bought from maplin, I think they're pretty cheap). If you can't measure any power while transmitting into the load then i think its reasonable to assume it's not transmitting. As suggested by the previous poster, check the wiring. If you have a multimeter that will measure current then put that in the supply and see if it is at least trying to draw some current when you transmit. Problem could be sticky relay (or is all the electronics solid state now) or dry joint. I had an intermittent transmit problem caused by a dry join in the past. The repair bill was £80, and all that was for for the labour of finding the dry joint, probably about 2 hours work, so if you can't fix it yourself it may be more economical to buy a new set.

Unless you're familiar with using an SWR meter, transmission line mismatch etc. you may be mislead by the SWR reading, and power reading, when you transmit into an antenna, especially if it has a fault.

If you carry an emergency VHF antenna can you do some comparison readings with that.

Do you have a friend on a boat next door that you can take the equipment to and test on their antenna?

Good luck.

 

[30boat]

Thanks all for the replies.I'll do another test but I'm pretty sure I'll end up buying a new set.Cheaper than getting it repaired.